This invention relates to reactors for chemical reduction of nitrogen oxide (NOx) emissions in the exhaust gases of automotive engines, particularly diesel and other engines operating with lean air fuel mixtures that produce relatively high emission of NOx. More particularly, the invention pertains to an improved method for preparing a non-thermal plasma reactor for use with diesel engines and the like.
In recent years, non-thermal plasma generated in a packed bed reactor has been shown to be effective in reducing nitric oxides (NOx) produced by power plants and standby generators. These units usually have a reducing agent, such as urea, to enhance the conversion efficiency. The packed bed reactor consists essentially of a high voltage center electrode inserted into a cylinder of dielectric material, usually a form of glass or quartz.
An outside or ground electrode is formed by a coating of metal in various forms, including tape, flame spray, mesh, etc. The space between the center electrode and the inside diameter of the dielectric tube is filled or packed with small diameter glass beads. When high voltage alternating current is applied to the center electrode, the surfaces of the beads go into corona, producing a highly reactive and selective surface for inducing the desired reaction in the gas.
Unfortunately, the packed bed design with its loose beads and glass dielectric is impractical for use in the conditions found in a mobile emitter, such as a car or truck. The vibration and wide temperature swings of the vehicle system would damage the packed bed and the necessary temperature and vibration isolation needed to make it survive would not be cost effective.
A reactor for use with diesel engines and other engines operating with lean air fuel mixtures is disclosed in commonly assigned U.S. patent application Ser. No. 09/465,073 (Attorney Docket No. DP-300477) entitled xe2x80x9cNon-thermal Plasma Exhaust NOx Reactor,xe2x80x9d which is hereby incorporated by reference herein in its entirety. Disclosed therein is a reactor element comprising high dielectric, nonporous, high temperature insulating means defining a group of relatively thin stacked cells forming gas passages and separated by the insulating means. Alternate ground and charge carrying electrodes in the insulating means on opposite sides of the cells are disposed close to, but electrically insulated from, the cells by the insulating means. The electrodes may be silver or platinum material coated onto alumina plates. Conductive ink is sandwiched between two thin nonporous alumina plates or other suitable insulating plates to prevent arcing while providing a stable electrode spacing for a uniform electric field. The electrodes are coated onto alumina in a pattern that establishes a separation between the electrodes and the connectors of alternate electrodes suitable to prevent voltage leakage.
In commonly assigned U.S. Provisional Application Serial No. 60/141,427 filed Jun. 29, 1999 (Attorney Docket No. DP-300505) entitled xe2x80x9cDesign and Method of Manufacturing a Plasma Reactor for Treating Auto Emissionsxe2x80x94Stacked Shapes,xe2x80x9d which is hereby incorporated by reference herein in its entirety, a non-thermal plasma reactor element is prepared from a planar arrangement of formed shapes of dielectric material, which shapes are used as building blocks for forming the region of the reactor wherein plasma is generated. The formed shape defines an internal cell in the plasma reactor having an exhaust passage for flowing exhaust gas to be treated therethrough. Individual cells are provided with a conductive print disposed thereon to form electrodes and connectors. In a preferred embodiment, the conductive print comprises a continuous grid pattern having a cutout region disposed opposite the terminal connector for reducing potential voltage leaks. Multiple formed cells are stacked and connected together to form a multi-cell stack.
Commonly assigned U.S. Provisional Application Serial No. 60/141,394 (Attorney Docket No. DP-300478) entitled xe2x80x9cPlasma Reactor Design for Treating Auto Emissionsxe2x80x94Durable and Low Costxe2x80x9d which is hereby incorporated by reference herein in its entirety, an non-thermal plasma reactor element for conversion of exhaust gas constituents is prepared from an extruded monolith of dense dielectric material having substantially planar internal features. The monolith comprises a plurality of channels separated by substantially planar dielectric barriers and a perimeter boundary wall. Conductive material printed onto selected channels forms conductive channels that are connected along bus paths to form an alternating sequence of polarity, separated by exhaust channels. Conductive channels and channels not selected for exhaust flow are plugged at end portions of the monolith with a material suitable for excluding exhaust gases and for preventing electrical leakage between conductive channels. During operation, exhaust gas flows through exhaust channels and is treated by high voltage alternating current flowing through the conductive channels. The substantially planar internal monolith features provide a uniform electrical response throughout the exhaust channels.
While the above non-thermal plasma reactors meet some of the current needs and objectives, there remains a need in the art for an improved, method for preparing such a reactor.
A method for preparing a non-thermal plasma reactor comprises forming a curved, swept-shaped substrate specifically designed for fabrication via extrusion and preparing a reactor element from the formed substrate. The method comprises forming the substrate, selectively coating the substrate, and firing and drying, as needed. The as-extruded curved substrate comprises a thick outer wall surrounding a plurality of channels separated by dielectric barriers. The curved substrate is formed from materials having a high dielectric constant such as, but not limited to, dense cordierite, alumina, titania, mullite, plastic, among others, or combinations thereof.
Selected channels are coated with conductive material to form conductor channels capable of forming an electric field around exhaust channels. Preferably, masking is employed to effect selective coating of the conductive material while preventing the conductive material from being applied to non-value added areas. A first mask is employed to apply conductive material forming conductor channels and bus paths. An additional mask may be employed to cover the conductor channels and bus paths with a sealant.
The prepared reactor element preferably comprises multiple concentric exhaust channels, multiple concentric conductor channels having alternating polarity, each connected to its respective polarity via bus paths, in-line structural support ligaments for providing optimal structural support while preventing exhaust leakage, and thick outer walls providing high crush resistance and allowing robust mounting into the reactor housing. The nested, concentric arrangement and curved shape substrate advantageously enhances the reactor""s ability to fit into vehicles. High durability is afforded by the thick outer walls, and in a preferred embodiment, the inclusion of integral structural support ligaments. The improved wall thickness achieved over prior ceramic plate designs provides the advantage of a more uniform electrical response. Improved resistance to voltage leakage is achieved by containing the channel conductors within dielectric channels (except at ends) and providing a dielectric coating at each end to prevent voltage leaks there.
The present method affords the advantage of minimizing manufacturing steps as compared to prior methods. For example, all of the substrate channels are coated at the same time. Further, firing cycles occur with the entire substrate rather than as multiple pieces coated and fired separately.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.